Handbook of Corrosion Engineering Episode 2 Part 8 doc

In acid solutions the anodicprocess of corrosion is the passage of metal ions from the oxide-freemetal surface into the solution, and the principal cathodic process is thedischarge of hy

10.1 Introduction

The use of chemical inhibitors to decrease the rate of corrosionprocesses is quite varied In the oil extraction and processing indus-tries, inhibitors have always been considered to be the first line ofdefense against corrosion A great number of scientific studies havebeen devoted to the subject of corrosion inhibitors However, most ofwhat is known has grown from trial and error experiments, both in thelaboratories and in the field Rules, equations, and theories to guideinhibitor development or use are very limited

By definition, a corrosion inhibitor is a chemical substance that, whenadded in small concentration to an environment, effectively decreasesthe corrosion rate The efficiency of an inhibitor can be expressed by ameasure of this improvement:

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TABLE 10.1 Some Corrosive Systems and the Inhibitors Used to Protect Them

indicates that inhibitor adsorption on metals is influenced by the lowing main features.

fol-Surface charge on the metal. Adsorption may be due to electrostaticattractive forces between ionic charges or dipoles on the adsorbedspecies and the electric charge on the metal at the metal-solutioninterface In solution, the charge on a metal can be expressed by itspotential with respect to the zero-charge potential This potential rel-ative to the zero-charge potential, often referred to as the (-potential,

is more important with respect to adsorption than the potential on thehydrogen scale, and indeed the signs of these two potentials may bedifferent As the potential of a metallic surface becomes more positive,the adsorption of anions is favored, and as the -potential becomesmore negative, the adsorption of cations is favored

The functional group and structure of the inhibitor. Inhibitors can also bond

to metal surfaces by electron transfer to the metal to form a coordinatetype of link This process is favored by the presence in the metal ofvacant electron orbitals of low energy, such as occurs in the transitionmetals Electron transfer from the adsorbed species is favored by thepresence of relatively loosely bound electrons, such as may be found inanions, and neutral organic molecules containing lone pair electrons or -electron systems associated with multiple, especially triple, bonds oraromatic rings The electron density at the functional group increases

as the inhibitive efficiency increases in a series of related compounds.This is consistent with increasing strength of coordinate bonding due

to easier electron transfer and hence greater adsorption

Interaction of the inhibitor with water molecules. Adsorption of inhibitormolecules is often a displacement reaction involving removal ofadsorbed water molecules from the surface During adsorption of amolecule, the change in interaction energy with water molecules inpassing from the dissolved to the adsorbed state forms an importantpart of the free energy change on adsorption This has been shown toincrease with the energy of solvation of the adsorbing species, which inturn increases with increasing size of the hydrocarbon portion of anorganic molecule Thus increasing size leads to decreasing solubilityand increasing adsorbability This is consistent with the increasinginhibitive efficiency observed at constant concentrations with increas-ing molecular size in a series of related compounds

Interaction of adsorbed inhibitor species. Lateral interactions betweenadsorbed inhibitor species may become significant as the surface cov-erage, and hence the proximity, of the adsorbed species increases.These lateral interactions may be either attractive or repulsive.Attractive interactions occur between molecules containing large

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hydrocarbon components (e.g., n-alkyl chains) As the chain length

increases, the increasing Van der Waals attractive force between cent molecules leads to stronger adsorption at high coverage.Repulsive interactions occur between ions or molecules containingdipoles and lead to weaker adsorption at high coverage

adja-In the case of ions, the repulsive interaction can be altered to anattractive interaction if an ion of opposite charge is simultaneouslyadsorbed In a solution containing inhibitive anions and cations theadsorption of both ions may be enhanced and the inhibitive efficiencygreatly increased compared to solutions of the individual ions Thus,synergistic inhibitive effects occur in such mixtures of anionic andcationic inhibitors

Reaction of adsorbed inhibitors. In some cases, the adsorbed corrosioninhibitor may react, usually by electrochemical reduction, to form aproduct that may also be inhibitive Inhibition due to the added sub-

stance has been termed primary inhibition and that due to the tion product, secondary inhibition In such cases, the inhibitive

reac-efficiency may increase or decrease with time according to whether thesecondary inhibition is more or less effective than the primary inhibi-tion Sulfoxides, for example, can be reduced to sulfides, which aremore efficient inhibitors

Effects of inhibitors on corrosion processes. In acid solutions the anodicprocess of corrosion is the passage of metal ions from the oxide-freemetal surface into the solution, and the principal cathodic process is thedischarge of hydrogen ions to produce hydrogen gas In air-saturatedacid solutions, cathodic reduction of dissolved oxygen also occurs, but foriron the rate does not become significant compared to the rate of hydro-gen ion discharge until the pH exceeds a value of 3 An inhibitor maydecrease the rate of the anodic process, the cathodic process, or bothprocesses The change in the corrosion potential on addition of theinhibitor is often a useful indication of which process is retarded.Displacement of the corrosion potential in the positive direction indi-cates mainly retardation of the anodic process (anodic control), whereasdisplacement in the negative direction indicates mainly retardation ofthe cathodic process (cathodic control) Little change in the corrosionpotential suggests that both anodic and cathodic processes are retarded.The following discussion illustrates the usage of anodic and cathodicinhibitors for acid cleaning of industrial equipment The combinedaction of film growth and deposition from solution results in foulingthat has to be removed to restore the efficiency of heat exchangers,boilers, and steam generators E-pH diagrams indicate that the foul-ing of iron-based boiler tubes, by FeO and FeO, can be dissolved in

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either the acidic or alkaline corrosion regions In practice, inhibitedhydrochloric acid has been repeatedly proven to be the most efficientmethod to remove fouling Four equations are basically needed toexplain the chemistry involved in fouling removal Three of thoseequations represent cathodic processes [Eqs (10.2) and (10.3); A, A′and A" in Figs 10.1 and 10.2; and Eq (10.4); B in Figs 10.1 and 10.2]and one anodic process [i.e., the dissolution of tubular material [Eq.(10.5); C in Figs 10.1 and 10.2]:3

Fe2O3 4 Cl 6 H 2 e→2 FeCl2(aq) 3 H2O (10.2)

Fe3O4 6 Cl 8 H 2 e→3 FeCl2(aq) 4 H2O (10.3)

Fe  2 Cl→FeCl2(aq) 2 e (10.5)These equations indicate that the base iron functions as a reducer toaccelerate the dissolution of iron oxides Because it is difficult to deter-mine the endpoint for the dissolution of fouling oxides, an inhibitor isgenerally added for safety purpose Both anodic and cathodic inhibitorscould be added to retard the corrosion of the bare metal after dissolution

of the fouling oxides Figures 10.1 and 10.2 illustrate the action thatcould be played by either an anodic inhibitor (Fig 10.1) or a cathodicinhibitor (Fig 10.2) It can be seen that although the anodic inhibitorretards the anodic dissolution of iron at the endpoint, it concurrentlydecreases the rate of oxide dissolution permitted by the chemical system

On the other hand, the cathodic inhibitor retards both the reduction

of protons into hydrogen and the dissolution of the base, whereas thereduction of the fouling oxides is left unaffected The E-pH diagramsalso indicate that the dissolution of the fouling oxides is also possible inalkaline solutions But the kinetics of anodic and cathodic reactions

in high pH environments are much slower, and therefore these tions are less useful

reac-Electrochemical studies have shown that inhibitors in acid solutionsmay affect the corrosion reactions of metals in the following main ways

Formation of a diffusion barrier. The absorbed inhibitor may form a face film that acts as a physical barrier to restrict the diffusion of ions

sur-or molecules to sur-or from the metal surface and so retard the rate of csur-or-rosion reactions This effect occurs particularly when the inhibitorspecies are large molecules (e.g., proteins, such as gelatin or agar agar,polysaccharides, such as dextrin, or compounds containing long hydro-carbon chains) Surface films of these types of inhibitors give rise toresistance polarization and also concentration polarization affectingboth anodic and cathodic reactions

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Blocking of reaction sites. The simple blocking decreases the number ofsurface metal atoms at which corrosion reactions can occur The mech-anisms of the reactions are not affected, and the Tafel slopes of thepolarization curves remain unchanged It should be noted that theanodic and cathodic processes may be inhibited to different extents.The anodic dissolution process of metal ions is considered to occur atsteps or emergent dislocations in the metal surface, where metalatoms are less firmly held to their neighbors than in the plane surface.These favored sites occupy a relatively small proportion of the metalsurface The cathodic process of hydrogen evolution is thought to occur

on the plane crystal faces that form most of the metal surface area.Adsorption of inhibitors at low surface coverage tends to occur prefer-entially at anodic sites, causing retardation of the anodic reaction Athigher surface coverage, adsorption occurs on both anodic and cathodicsites, and both reactions are inhibited

Participation in the electrode reactions. Corrosion reactions ofteninvolve the formation of adsorbed intermediate species with surfacemetal atoms [e.g., adsorbed hydrogen atoms in the hydrogen evolu-tion reaction and adsorbed (FeOH) in the anodic dissolution of iron]

Figure 10.1 The effect of an anodic inhibitor on the dissolution rate of iron and iron oxide 3

The presence of adsorbed inhibitors will interfere with the formation

of these adsorbed intermediates, but the electrode processes maythen proceed by alternative paths through intermediates containingthe inhibitor In these processes the inhibitor species act in a cat-alytic manner and remain unchanged Such participation by theinhibitor is generally characterized by an increase in the Tafel slope

of the anodic dissolution of the metal

Inhibitors may also retard the rate of hydrogen evolution on metals

by affecting the mechanism of the reaction, as indicated by increases inthe Tafel slopes of cathodic polarization curves This effect has beenobserved on iron in the presence of inhibitors such as phenyl-thiourea,acetylenic hydrocarbons, aniline derivatives, benzaldehyde derivatives.and pyrilium salts

Alteration of the electrical double layer. The adsorption of ions or speciesthat can form ions on metal surfaces will change the electrical doublelayer at the metal-solution interface, and this in turn will affect therates of the electrochemical reactions The adsorption of cations, such asquaternary ammonium ions and protonated amines, makes the poten-tial more positive in the plane of the closest approach to the metal of

Figure 10.2 The effect of a cathodic inhibitor on the dissolution rate of iron and iron oxide 3

cathodic reaction in neutral solutions is the reduction of dissolved gen, whereas in acid solution it is hydrogen evolution Corroding metalsurfaces in acid solution are oxide-free, whereas in neutral solutionsmetal surfaces are covered with films of oxides, hydroxides, or salts,owing to the reduced solubility of these species Because of these differ-ences, substances that inhibit corrosion in acid solution by adsorption onoxide-free surfaces do not generally inhibit corrosion in neutral solution.Typical inhibitors for near-neutral solutions are the anions of weakacids, some of the most important in practice being chromate, nitrite,benzoate, silicate, phosphate, and borate Passivating oxide films onmetals offer high resistance to the diffusion of metal ions, and theanodic reaction of metal dissolution is inhibited These inhibitiveanions are often referred to as anodic inhibitors, and they are more

Figure 10.5 Corrosion of AISI 1018 carbon steel in 6 M HCl containing 1000 ppm trans-cinnamaldehyde.

TABLE 10.2 Inhibitor Efficiency of Trans-Cinnamaldehyde (TCA) to the

Corrosion of Carbon Steel Exposed to a 6 M HCl Solution

Corrosion current, Corrosion rate,

generally used than cathodic inhibitors to inhibit the corrosion of iron,zinc, aluminum, copper, and their alloys in near-neutral solutions Theaction of inhibitive anions on the corrosion of metals in near-neutralsolution involves the following important functions:

1 Reduction of the dissolution rate of the passivating oxide film

2 Repair of the oxide film by promotion of the reformation of oxide

3 Repair of the oxide film by plugging pores with insoluble pounds

com-4 Prevention of the adsorption of aggressive anions

Of these functions, the most important appears to be the stabilization

of the passivating oxide film by decreasing its dissolution rate tion 1) Inhibitive anions probably form a surface complex with themetal ion of the oxide (i.e., Fe3 , Zn2 , Al3 ), such that the stability ofthis complex is higher than that of the analogous complexes withwater, hydroxyl ions, or aggressive anions

(func-Stabilization of the oxide films by repassivation is also important(function 2) The plugging of pores by formation of insoluble com-pounds (function 3) does not appear to be an essential function but isvaluable in extending the range of conditions under which inhibitioncan be achieved The suppression of the adsorption of aggressiveanions (function 4) by participation in a dynamic reversible competi-tive adsorption equilibrium at the metal surface appears to be related

to the general adsorption behavior of anions rather than to a specificproperty of inhibitive anions

Inhibition in neutral solutions can also be due to the precipitation ofcompounds, on a metallic surface, that can form or stabilize protectivefilms The inhibitor may form a surface film of an insoluble salt by pre-cipitation or reaction Inhibitors forming films of this type include

■ Salts of metals such as zinc, magnesium, manganese, and nickel, whichform insoluble hydroxides, especially at cathodic areas, which are morealkaline due to the hydroxyl ions produced by reduction of oxygen

■ Soluble calcium salts, which can precipitate as calcium carbonate inwaters containing carbon dioxide, again at cathodic areas where thehigh pH permits a sufficiently high concentration of carbonate ions

■ Polyphosphates in the presence of zinc or calcium, which produce athin amorphous salt film

These salt films, which are often quite thick and may even be visible,restrict diffusion, particularly of dissolved oxygen to the metal surface.They are poor electronic conductors, and so oxygen reduction does not

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occur on the film surface These inhibitors are referred to as cathodicinhibitors.

The following sections discuss the mechanism of action of inhibitiveanions on iron, zinc, aluminum, and copper

Iron. Corrosion of iron (or steel) can be inhibited by the anions of mostweak acids under suitable conditions However, other anions, particu-larly those of strong acids, tend to prevent the action of inhibitiveanions and stimulate breakdown of the protective oxide film Examples

of such aggressive anions include the halides, sulfate, and nitrate Thebalance between the inhibitive and aggressive properties of a specificanion depends on the following main factors (which are themselvesinterdependent):

■ Concentration. Inhibition of iron corrosion in distilled water occursonly when the anion concentration exceeds a critical value At con-centrations below the critical value, inhibitive anions may actaggressively and stimulate breakdown of the oxide films Effectiveinhibitive anions have low critical concentrations for inhibition Anumber of anions have been classified in order of their inhibitivepower toward steel, judged from their critical inhibitive concentra-tions The order of decreasing inhibitive efficiency is azide, ferri-cyanide, nitrite, chromate, benzoate, ferrocyanide, phosphate,

tellurate, hydroxide, carbonate, chlorate, o-chlorbenzoate,

bicarbon-ates fluoride, nitrate, and formate

■ pH. Inhibitive anions are effective in preventing iron corrosiononly at pH values more alkaline than a critical value This critical

pH depends on the anion

■ Dissolved oxygen concentration and supply. Inhibition of the sion of iron by anions requires a critical minimum degree of oxidiz-ing power in the solution This is normally supplied by the dissolvedoxygen present in air-saturated solutions

corro-■ Aggressive anion concentration. When aggressive anions are sent in the solution, the critical concentrations of inhibitive anionsrequired for protection of iron are increased It has been shown thatthe relationship between the maximum concentration of aggressive

pre-anion Cagg permitting full protection by a given concentration of

inhibitive anion Cinhis of the form

log Cinh n log Cagg K where K is a constant dependent on the nature of the inhibitive and aggressive anions, and n is an exponent that is approximately the

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ratio of the valency of the inhibitive anion to the valency of theaggressive anion

■ Nature of the metal surface. The critical concentration of an anionrequired to inhibit the corrosion of iron may increase with increas-ing surface roughness

■ Temperature. In general, the critical concentrations of anions (e.g.,benzoate, chromate, and nitrite) required for the protection of steelincrease as the temperature increases

Zinc. The effects of inhibitive and aggressive anions on the corrosion

of zinc are broadly similar to the effects observed with iron Thus withincreasing concentration, anions tend to promote corrosion but maygive inhibition above a critical concentration Inhibition of zinc corro-sion is somewhat more difficult than that of iron (e.g., nitrite and ben-zoate are not efficient inhibitors for zinc) However, inhibition of zinccorrosion is observed in the presence of anions such as chromates,borate, and nitrocinnamate, which are also good inhibitors for the cor-rosion of iron Anions such as sulfate, chloride, and nitrate are aggres-sive toward zinc and prevent protection by inhibitive anions Thepresence of dissolved oxygen in the solution is essential for protection

by inhibitive anions As in the case of iron, pressures of oxygen greaterthan atmospheric or an increase in oxygen supply by rapid stirring canlead to the protection of zinc in distilled water Inhibition of zinc cor-rosion occurs most readily in the pH range of 9 to 12, which corre-sponds approximately to the region of minimum solubility of zinchydroxide

The ways in which inhibitive anions affect the corrosion of zinc aremainly similar to those described above for iron In inhibition by chro-mate, localized uptake of chromium has been shown to occur at lowchromate concentrations and in the presence of chloride ions.Inhibitive anions also promote the passivation of zinc (e.g., passivation

is much easier in solutions of the inhibitive anion, borate, than in tions of the noninhibitive anions, carbonate and bicarbonate) A criti-cal inhibition potential, analogous to that on iron, has been observedfor zinc in borate solutions Thus inhibitive anions promote repair ofthe oxide film on zinc by repassivation with zinc oxide

solu-Aluminum. When aluminum is immersed in water, the air-formedoxide film of amorphous -alumina initially thickens (at a faster ratethan in air) and then an outer layer of crystalline hydrated aluminaforms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminum is generally inhibited

by anions that are inhibitive for iron (e.g., chromate, benzoate,

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